text:
Why is Russian super-maneuverable thrust vectoring more prevalent than American?
Based on the sheer number of super-manouverable thrust vectoring Russian fighter jets, it seems that Russia has put more effort into developing thrust vectoring for enhanced maneuverability than the US has.  Why is this?
Examples of American thrust vectoring aircraft:

F-22 Raptor

Russian thrust vectoring aircraft:

Su-30 MKI
Su-35
PAK FA
Su-37
Mig-35

Of course you could include the MV-22 Osprey, Harrier, and F-35B as thrust vectoring, but I'm talking about thrust vectoring for enhanced maneuverability, not for STOVL.

Have you tried to research the requirements against which the Russian jets were produced/procured by their air force?

Historically, Soviet/Russian jets have sacrificed firepower for maneuverability, while US jets sacrifice maneuverability for firepower

@Timpanus tell that to pilots of MiG-23s, MiG-25s, Mig-31s, Su-15s, F-16s, F-15s, F-86s... In both cultures there are trends going one way or the other over time

Part 1 - TVC
Perception
The US also ran several programs with TVC:

F-15 STOL/MTD and ACTIVE.

F-16 VISTA / MATV. AVEN:

F-18 HARV (High Alpha Research Vehicle). Overview pdf.

F-22 (almost 200 produced)
X-31.

X-36. (Yaw only.)

X-44 MANTA (proposed F-22 variant)

(All photos from their respective wiki pages except where otherwise watermarked.)
The F-22 is far from the sole example of US TVC, though the only design to enter serial production.
From the Russian side, you mention the "sheer number of super-maneuverable thrust vectoring Russian fighter jets":

Su-30 MKI
Su-35
PAK FA
Su-37
MiG-35

But the PAK FA is still deep in development, the MiG-35 probably won't enter production was delayed, and the Su-37 never entered production. To date, only a handful of those three were ever built.
The Su-30MKI was built for India, not Russian forces (see Su-30 for Russian variant). It doesn't use true 3D TVC, but rather fixed 3D TVC: . Explained here:

The Su-35 is the only one [on that list] in widespread Russian service, and only a few dozen of those were produced.
Also notice that the Su-30/35/37 are all part of the same Su-27 family. In fact, the Su-37s were just converted Su-35s. And the MiG-35 is derived from the MiG-29. So there isn't a preponderance of clean sheet designs built with TVC.
In comparison, the US list above includes four different manufacturers and six clean-sheet designs (plus only one derivative), half of which included original TVC solutions (rather than merely retrofitted).
Conclusion
The US had a number of TVC programs itself.
So why does Russia seem like it "put[s] more effort" into TVC?
Beyond flying more TVC fighters, it's also more popular in the news. The Russians have always liked to talk big about their "secret wonder weapons," and US media doesn't discourage them. People devour headlines like "Russian fighters out-turn F-22, US stealth is a hoax, newest US fighter is a turkey." It sells pretty well.
Real design is complex, hard to understand, and easy to deliberately misinterpret.
In contrast, the newest US fighter, the F-35, doesn't have TVC. Since it's A) the only new US fighter since the Super Hornet (other than the aforementioned F-22) and B) replacing so many a/c (F-16, F-18, AV-8B, and probably some A-10s), it means that much of the US fighter inventory won't have TVC. On the other hand, had the JSF program produced 3 completely separate a/c, at least one of those may have included TVC (though unlikely in my view).
Also, Russian TVC employment is ongoing and fresher in people's minds, whereas the US projects above are a bit older and have apparently fallen out of the public consciousness.

So why does Russia use it more than the US?
Russia developed its Su-27 and MiG-29 families in response to the F-15, F-16, and F-18, which were a quantum leap in capability over their predecessors, like the F-4, F-5, Mirage, and Century Series. The F-16 had great sustained turn rate, and the F-15 had powerful engines on a big wing.

But their first engagements with F-15s left the Mirage pilots shaking
  their heads. One Mirage ace with fourteen kills described his first
  fight with an F-15 whose pilot was just out of F-15 training school to
  the author. "The rules were that he could not use his AIM-7s, so the
  fight began with a head-on pass. I started to turn and he pulled up
  and came around on me. I saw him make three or four mistakes on the
  way that I could have easily taken advantage of if he had been in a
  regular fighter, but there was nothing I could do to counter the F-15.
  He shot me down within forty seconds. I flew home in my Mirage, both
  of us feeling very old and out of date." -- Revolt of the Majors: How
  the Air Force Changed After Vietnam, by Marshall L. Michel III

The Su-27 was designed to be a bit better. TVC helped to achieve that [or at least tried to].
TVC is useful at very low speeds and high alpha, where conventional control surfaces are less effective. So it's useful for dogfighting---close range, low speeds, lots of turning.
However, using TVC comes at a cost in forward thrust (and drag and weight), which the video above explains. Outside that envelope (where the control surfaces work fine), TVC doesn't see much use.
Unfortunately for TVC, engagement ranges were growing, and within visual range combat was becoming less common, so those kinds of performance parameters were losing utility. New technologies and tactics made it very difficult to get close in the first place: AWACS, improved fighter radars (greater range, look down/shoot down), BVR ID (non-cooperative target recognition), data-fusion, networks, and new long range missiles (higher reliability, improved countermeasure resistance, fire and forget, datalink). BVR combat finally came of age.
Even if you made it to close range, new dogfighting missiles made combat very lethal (see: HOBS, all-aspect engagement envelope, LOAL, HMCS, IIR seekers).
Long-range wasn't the only thing that sidelined TVC. Other designs could also provide control at low speed and high AOA without TVC. The euro-canards are basically supermaneuverable. The F-35 is controllable to ~50 deg AOA and was successfully tested to 110 degrees:

You'll remember that the YF-23 (competitor to the YF-22) doesn't feature TVC. It counted more on higher speeds and better stealth to help keep adversaries at arms' length, rather than tangoing at low speed with TVC. (But note that the YF-23 was also very maneuverable, with a higher sustained turn rate than the F-16, as required.)
So while the US did not neglect TVC, it did opt for different designs and doctrines that had much less use for TVC. It didn't need TVC to attain performance goals, and it doesn't plan on needing that kind of performance very often in the first place.
Now, once the Cold War ended, Russian defense budgets were slashed, and funding for new a/c development dried up, so Russian forces haven't fielded a brand new fighter for over two decades. At least not in meaningful numbers. They're still using the Su-27 and MiG-29 families from pre-1991, just newer variants thereof. So they're not so much 're-emphasizing' super-maneuverability as they are still 'stuck' on it.[That word isn't quite right, but I can't think of a better one.]

Part 2 - Responding to other answers
Please pardon my pet peeves.
1. The F-18 "[emphasizes] maneuvering energy-efficiently, over expending all your energy in tight turns to simply 'get a bead' on your bogey first."
That depends. Compared to an F-16, the Hornet has a lower sustained rate of turn but a much higher AOA (Hornet = 50, F-16 = 15-26.5 degrees, depending on g and loading). The Hornet should never win a fight against an F-16. But it does. Sometimes.
The Hornet can yank its nose, spending that energy, to get the first shot. But if it misses, the F-16 can gradually work its way into a firing position. Rate v. radius.
2. E-M theory as a revolution
Boyd is credited with a lot of things. Humility is not one of those things. Embellishment and self-promotion are. The cult of the man has grown to such proportions in the popular conception that it's difficult to critique him and his ideas without becoming a lightning rod.
While E-M was useful for summarizing some principles of aerial maneuver, it did not invent those principles. Arguably, its real utility was being able to quantify paper designs, not just metal ones.
3. E-M theory in a/c design. "F-15, F-16, and F-18 are designed specifically around energy maintenance in all flight regimes"
I wouldn't say they were "designed around" E-M. It was used to evaluate the designs (after they were already designed), sure, but so were many other metrics.
For example, Boyd wanted the F-15 limited to 5.5 g's because that's what his E-M charts said was most efficient. Needless to say, that didn't happen.
Kinematics are only one aspect of fighter design. Avionics are often more important.
In Korea, F-86s went 10:1 against MiG-15s. Both had the same thrust, but the F-86 weighed significantly more than the MiG, lowering its acceleration/climb rate and altitude.

[Korean fighters aces and Doolittle] felt that the high-technology avionics, while heavy and hard to maintain, allowed the Air Force F-86s to have the high kill ratio despite the Soviet fighters lighter weight. Indeed, Soviet MiG-15 pilots who later examined downed F-86s were extremely envious of the very F-86 systems Boyd and others criticized. --- Revolt of the Majors, 80

It's arguably more accurate to say the F-15 was built around its complex, expensive radar, which Boyd opposed, having never flown an a/c with either radar or missiles.
Boyd was unhappy with both the F-15 and F-16, especially with what they became: high-tech.
4. "US aircraft development has pretty much stopped cold in the mid-late 1980s..."
The F-35 is a HUGE leap over its predecessors (F-16, F-18, AV-8B, F-117): stealthy AESA radar; advanced EW suite; advanced, spherical missile approach and warning; new helmet mounted cueing system and display; integrated FLIR, laser designator/range-finder, and IRST; stealthy, high data rate comms; longer range; DSI; ALIS; lower maintenance electrohydrostatic actuators; lower maintenance, more durable, "baked in" stealth coatings; tightly integrated and fused sensors.
In some ways, the F-35 is more advanced than the F-22. For example, the radar, EW suite, and MAWS are more modern versions of those on the F-22. Some of the F-35's technologies, like coatings and software, will be retrofitted onto the F-22.
5. "[Only] the F-22 and F-35 [entered] production since [the mid-late 1980s]"
The F-22 and F-35 aren't the only US a/c that entered production in the last 26 years.

C-17 entered production in 1991/92.
V-22 entered full-rate production in 1995.
The Super Hornet entered production in 1995.
Growlers entered production in 2007.
P-8 Poseidon entered production ~2009.
E-2D saw initial deliveries in 2010.
KC-46 entered production a few years ago.
T-X (new trainer) will likely start production soon.
JSTARS is looking for a replacement.
On the helo side, there's KMAX and the proposed S-97.
The B-21 Raider ("mini B-2") is deep in development.
And let's not forget all the UAVs:
 (Dronesurvivalguide.org)

And there's also the less public ones, like the Doritos over Texas.
Also, "just" the F-35 is understating things a bit, since its 3 variants are intended to replace several different fighters (F-16, F-18, AV-8B, F-117, and probably some A-10s) and over 2,500 airframes in US inventory.
6. The F-22 and F-35 are "decades overdue"
They've only been in development for 15-20 years (depending on how you count), so they can't be "decades" overdue. This is average for modern fighters:

The F-35’s development timeline is also relatively moderate. The Rafale started development in 1982 and introduced in 2001(19 years). The Euro-fighter started in 1983 and was introduced in 2003(20 years). The Raptor started in 1986 and entered service in 2005(19 years). The PAK-FA, an evolutionary aircraft, started in 2001 and will be introduced in 2017(16 years). The Gripen started development in 1979 and was introduced in 1998(19 years). The Hornet was a redesign of the YF-17(9 years) from 1975 to 1983(8 Years) from which the Super Hornet evolved from 1992 to 2000(8 years). The F-35 in comparison to all of these started in 1996 with USAF IOC in 2016(20 years). Source.

7. "...TV simply wasn't ready for operational deployment."
The US experimented with TVC a number of times, but [I'd argue] they deliberately didn't employ it in the "current generation" (ie F-35) because the cost, complexity, weight, and increased maintenance weren't worthwhile for the rare times they'd be useful.
CORRECTION: I misunderstood what s/he meant by "current operational generation." S/he meant F-15/16/18, not F-22 onwards. So they're right that TVC definitely wasn't ready for the F-15/16/18. Otherwise it might have been incorporated into those designs.

"The Russians have always liked to talk big about their "secret wonder weapons," and US media doesn't discourage them. " A bit of a russophobe are we?
A few examples of mistakes in your "facts"-
"the MiG-35 probably won't enter production" Mig-35s are in production. Orders for Egypt and Russian Air Force ~90 aircraft.
"PAK FA is still deep in development" Sure, with first aircraft to enter service in 2017.."In Korea, F-86s went 10:1 against MiG-15s" More like 5,6:1..

@MathematicianByMistake - It's not a criticism. It's an understandable part of its procurement system, somewhat akin to commercial marketing material. Sometimes. Other times it's propaganda and misinformation, of which there are numerous examples from the both Cold War and today (remember the 10,000 km range "robotic" nuclear torpedo that would create "wide areas of radioactive contamination", which was "accidentally" leaked to Russian state media?). The Russians made good gear and played a good game---that doesn't require them to talk straight.

@MathematicianByMistake - F-86/MiG-15 kill/loss ratio. I'm assuming you got the 5.6:1 figure from Dildy and Thompson via TNI? I haven't read their book, so I can't comment on their work or methodology. For now, I'm going to go with the US Air Force and RAND. For now at least.

@MathematicianByMistake - By any measure, the PAK FA is still in low-rate initial production and undergoing testing and evaluation. Even very recently, the engines were still annoyingly bare. It's real engines won't be installed for a couple years (2020 at the earliest?). Wrapping up development would be harder with full Indian support. But maturing the systems, especially avionics, will take time (as it should) and shouldn't be rushed.

@MathematicianByMistake - I justed checked. You may very well be right about new MiG-35 orders. I haven't been following the program ever since the Russian Ministry of Defense postponed its earlier order due to structural budget cuts. The Indians also passed up the MiGs in favor of 36 Rafales (I believe they finalized that deal last year). So I wasn't terribly optimistic. You're absolutely right about the Egyptian sale, though. It's even in the wiki :/

This is a case of divergent design, and you can't know the answer without asking why current aircraft are designed they way they are.
Following the work of Col. John Boyd in the 1960's in developing the 'Energy-Maneuverability Theory', the U.S. and allies primarily designed combat aircraft and tactics around the idea that an aircraft's specific energy state defines its performance during basic fighter maneuvering (dogfighting).
Under the E-M theory, in order for a fighter to be survivable (and consequently victorious) during aerial combat, it must maintain a high specific energy advantage throughout its maneuvering - this means it either has higher potential energy (altitude), or higher kinetic energy (speed), and should aim to maintain the capacity to trade one for the other efficiently. In a fight occurring only in the vertical plane, this is easy to quantify, but in a turning fight, induced drag from turning bleeds energy from the aircraft, and E-M theory allows this performance efficiency to be quantified for a particular design or tactic.
Consequently, U.S. 4th Gen fighters like the F-15, F-16, and F-18 are designed specifically around energy maintenance in all flight regimes - high thrust for climb performance (gaining potential energy via altitude) and to maintain speed (overcome induced drag by turning), low wing loading (lower energy bleed in turning due to induced drag), and improved aerodynamics in the expected combat envelope (reducing overall drag in the subsonic regime), and their tactics emphasize maneuvering energy-efficiently, over expending all your energy in tight turns to simply 'get a bead' on your bogey first.
The practical effect of this is that if your first attack pass fails, you still have sufficient energy to either outmaneuver your opponent and get into position for another attack, or run. The disadvantage is, when your kinetic energy is higher than your opponents (speed) you can't turn as tightly as he can - but the hope is your greater ability to maneuver in 3 dimensions with that energy will give you the upper hand for a second attack - classically, this would be considered 'boom and zoom' tactics.
By contrast, the Russian designers have been focusing on 'Supermaneuverability', the ability of the aircraft to remain within the controllable envelope, outside the envelope of aerodynamic control. This is facilitated by 'funny' aerodynamics on aircraft like the MiG-29, Su-27 and Su-33, and by thrust vectoring on the aircraft you mention in the OP. This allows for high angle-of-attack maneuvering at low speeds (low energy state) - with the goal being to allow the pilot to point their aircraft at their opponent to facilitate a weapon release - it's the 'shoot first' strategy. Even if your shot misses, you force your opponent to go defensive, giving you the upper hand for a second shot, often facilitated by high off-boresight missiles like the R-73.
As speed decreases, so does turning radius - however aerodynamic control authority decreases at a greater rate. Thrust vectoring allows you to overcome the decreased aerodynamic control authority - your ability to change the attitude of the aircraft is now mostly independent of its aerodynamic control surfaces. Thus using supermaneuverability, you no longer need to outturn your opponent, you just point the aircraft in a totally different direction to where you were flying, possibly allowing a 'snap shot' with missiles or guns. The classical equivalent is 'turn and burn' tactics.
The practical effect of this strategy is that you might outturn your target and get a shot off first, but if that shot fails, you now have no energy (because you bled it off due to the massive induced drag of a high-alpha turn), and are effectively a sitting duck to your opponents wingman - you have no speed to run, you're losing altitude, all you have is thrust, and thrust is comparatively slow to regain energy. You can spin in place and keep shooting, but if your opponent uses their energy to run, you don't have the energy to chase.
So, why has the U.S. primarily ignored thrust vectoring? Because it's not tactically relevant to them. The West experimented with supermaneuverability towards the end of the Cold War with aircraft like the X-31 and F-15 ACTIVE, and after getting their hands on MiG-29's and Su-27's at the end of the Cold War, and wargaming them, decided that the 'supermaneuverability' advantage just wasn't worth it, when E-M based tactics could simply be adapted to defeat it. The one exception being the F-22 - an aircraft designed before the Wall fell, specifically designed to counter what Western tacticians had imagined the MiG-29 and Su-27 were capable of - illusions that were shattered just several years later.

Welcome to the site!

Wow.  Thanks for explaining this so thoroughly.

I would take great care in ascribing so much credit to Boyd. While popular and easily understood, his ideas were neither revolutionary nor the critical determinants for the design of the F-15, F-16, or F-18. Indeed, his ideas most often ran counter to what those aircraft would become.
Also, his wiki page reads like an advertisement.

A very important reason is stagnation.
US aircraft development has pretty much stopped cold in the mid-late 1980s, with just the F-22 and F-35 entering production since, and those decades overdue.
Meanwhile, the  Soviets created and fielded all the types you mention. At the time the current operational generation of US aircraft entered service, TV simply wasn't ready for operational deployment. And remember that the USSR/Russia has always fielded a wider variety of combat aircraft than does the US, typically having multiple types in production at the same time for the same job.
All the aircraft you mention are of the same generation, built after the last new US aircraft prior to the F-22 (the F-15) entered production or even prototype stage.
The US experimented with a thrust vectoring variant of the F-15, and another one based on the F-16 as well, but both projects fell foul of budget cuts when Clinton took over from Reagan and no orders were ever placed. In the Soviet system most things that weren't total failures would see at least some models produced and put into operational service. A far more expensive way of doing things, but fitting into their political/economic system.

Please leave a blank line between paragraphs, rather than using HTML line breaks. Reducing the wall of text makes the post much more readable.

The “new” Russian designs are not really new. They are upgrades to the older designs, mainly with improved avionics, though it's still mostly catching up with the west (e.g. MiG-35 will be the first Russian fighter using an AESA radar, something US and European aircraft use for over a decade). But not much new on the airframes. Meanwhile, USA fielded the F-22, which is a completely new design. And it has huge advantages: if the F-22 can fire at you from almost 100 miles (nautical) away without you having slightest clue it's there, no thrust vectoring in the world is going to help you.

I don't think the Soviets developed new aircraft much later than the late 80s, either... Also, Clinton took over from George H. W. Bush, not Reagan, though Bush's policy was probably largely an extension of Reagan's, since he was Reagan's VP.

The F-22 and F-35 were developed over 15-20 years (depending on how you count), so it's hard to say they were "decades" overdue.


text:
Who controls the cabin air temperature?
I understand the B737-800 has two air conditioning packs that handle the temperature of three zones: the cockpit, the fwd cabin and the aft cabin.
I understand that which pack is turned on is controled from the cockpit.
My questions are:

can the temperature be regulated for each zone seperately?
where are the switches located that control the temperature in each zone?
who operates these switches?

Temperature selectors shown in answer to Do pilots breathe the same air as passengers?

Your understanding is correct re: who controls it. See here
https://aviation.stackexchange.com/questions/24809/why-is-air-mixed-with-bypass-air-on-the-a-c-of-an-aircraft

The 737-400, -800, and -900 have two cabin air conditioning zones plus the cockpit. The rest have one plus the cockpit.
Both packs are on in normal operation because they are slightly more efficient at lower flow rates. It also is safer as no immediate action is necessary to prevent depressurization if a pack fails.
Temperature is regulated separately. The packs cool to the lowest temperature demanded, and unconditioned bleed air, called trim air, is mixed in for the warmer zone.
The dual cabin zone models have three knobs which set the temperature of each zone, from a low to high temperature with a default detent.
The single cabin zone models have a knob for the cabin and one for the cockpit zone. The knob is divided into two ranges, an auto range, where a temperature is controlled, and a manual range, that selects the position of the hot air mix valve in each pack.
The temperature control dials are located in the overhead panel in the cockpit. The pilots operate this, often in response to complaints from the cabin crew relayed over the interphone. (The A320 has an option for cabin crew adjustment of temperature, eliminating the hassle)

So can this trim air (and hence temperature) be regulated for each zone separately? Can a flight atendant ask the flight crew to adjust e.g. only the fwd cabin temperature?

Ah, I see in the link from @fooot that there are separate dials for each zone. Thank you all very much.

@PaulOgilvie Updated my answer slightly to describe the knobs.

If I may add one more question: am I correct to understand the temperature setting(s) manage the temperature of the air provided to the cabin, but there are no temperature sensors in the cabin that regulate this output air? So a cold hull will not receive hotter air, even though the heat is already "consumed" after the first few rows?

@PaulOgilvie No. The 737 has one temperature sensor per zone, which measures cabin air temperature independent of output air. Just like a home thermostat. The heating isn't "consumed": air ventilation systems on aircraft are designed not to have much forward-aft flow.

Thank you. I think my understanding of cabin heating is now complete (nearly).

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
Where does the gate drop-off bagage go?
Sometimes I am asked to give-off my cabin luggage case at the gate and it will be returned to me upon arrival. I often don't mind, as long as I do get it back at the gate upon arrival and don't have to go to the baggage pick-up belts. Is this luggage stored in some particular place (so I can be more assured the gate drop-off won't delay me)?

@ymb1, I'll subscribe to travel.se.

It's typically stored in the cargo bin with the checked luggage.  If it is to be returned to the jetway, it will be tagged with a distinctive tag so that it's clear that it has been screened already (i.e. it came in through security, so it can go back out into the terminal with you at your destination).  In many cases, though, once an item has been gate-checked, it will be delivered to baggage claim to be picked up there.
The exception to that is baby strollers -- they should be returned to the jetway since parents tend to need those to get the young ones around the airport.
Individual airline policies, of course, will vary.  With small jets (not a 737, but a regional jet or turboprop, perhaps), the carry-on space is so limited that all sorts of things that would be accepted as a carry-on for a larger aircraft can't go into the cabin, so those may go into a dedicated cargo hold & all returned as the passengers deplane.

Thanks @Ralph-j. I'll disguise my cabin luggage case as a baby stroller :-)

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Compressor-Turbine: Why a shaft and not kind of a "rotating drum"?
I'm studying the fundamentals of jet propulsion and its applications. I really have this next question and haven't found an answer yet: why do jets use a shaft (or more) to connect the turbine(s) to the compressor device(s), but not a "rotating drum" instead?
What I call a "rotating drum" (I sketched it below) is a thin, rotating cylinder that has built-in rotor blades at the beginning and exit.
I'd like to know if this is a feasible arrangement, and what would be the disadvantages of it. I think this might be unfeasible because:

There would be a lot of thermal stress at the middle of the drum.

But if we were to overcome this drawback somehow, I think we would benefit in the following ways:

Less weight, I think, with respect the traditional shaft system.
More internal space (no hollow) for other demanding components, like the combustor. The combustor could be made a bit larger, thus increasing the burning efficiency (more reaction time for reactants, ...).

I don't know how correct/wrong I am. This is a question out of pure curiosity and I wanted to get some feedback on it. Should I go ahead and start working on a small scale prototype of it? Hope my question here is not too basic.
PS: The sketch I made is just that: a sketch. Therefore, important things like the relative number of stages compressor-turbine, or the increasing/decreasing areas for each of those two components, do not appear.

The drum would still need support and bearings between its rotating part and the support structure. The likely place for this is in the center where a shaft would go.

Either that, or a "guide" in the center, that impeded the horizontal move and where bearings would be placed.

If I understand your drawing correctly, the "turbine" and "compressor" portions are fixed and the "rotating cylinder" portion spins around it, and the pieces labeled "S" would be the stators or guide vanes? In that case, the bearing surfaces would be the tops of the stators and the "lands" between the rotor blades. I can see basic value in this for a single shaft engine, but it seems that multi-shaft engines are the future. Interesting concept, nice drawing, and excellent hand writing!

Thanks FreeMan, too kind of you! For multi-shaft engines we could basically think of using coaxial rotating drums (but I'm not sure of that). Your understanding of the sketch was very precise.

Centripetal force will tear the drum apart.

That's mostly how a CFM56-7B, one of the most common turbofans, works. The N2 shaft is actually a drum with two conical ends that rotate around the actual N1 shaft. There must be a shaft as soon as their are two coaxial spools. See this answer for more details. The HP stators are fixed to the engine case (contrary to your drawing, the stators are the outer components).

There are a few very practical reasons why turbine engines use inner shafts/spools.
First, jet engine spools are very high speed rotating parts, this means they are subjected to high amounts of centripetal force. Centripetal force is calculated by $F=mr\omega ^2$ were F is Force, m is mass, r is radius of gyration and $\omega$ is angular velocity. So obviously the larger the radius of gyration the higher the forces that the rotors must carry. To reduce the radius of gyration manufacturers try to push the mass of the rotors as close to the centerline of the engine as possible. In the picture below (source) I have circled the HPC, HPT and LPT rotors. You can see how they are very thin under the blades and then have a large bulb very close to the engine centerline. Moving all of this mass towards the centerline, lowers the radius of gyration, and thus reduces the centripetal force. By reducing the centripetal force, you can reduce the strength of the rotors as well, since they don't have to carry as much load. This allows you to remove mass from the rotors, further reducing the centripetal force and overall weight of the engine.

Next, you want the bearings to be as small as possible. This is because as the radius of a bearing increases the linear velocity of it also increases, via this equation: $v=\omega r$. The faster the linear velocity the higher the wear, the greater the friction and greater the heat generation. So there is a push to make the bearings as small as possible given other constraints. In the image below I have circled the bearings, the red circles are on the low pressure spool bearings, while the blue circles are the high pressure spool bearings. This isn't always done, but it is becoming very common for the aft high pressure bearing to actual ride on the low pressure shaft. Typically these shafts spin in the same direction, so the bearing speed is reduced by this equation $\omega_{AftHPbearing}=\omega_{HP}-\omega_{LP}$. Bearing friction reduces the efficiency of the engine, and bearing wear is a significant maintenance driver.

(This is getting long so I am going to truncate the rest) There would be many other issues as well, such of blade mounting (dovetails in compression instead of tension), blade design (compression instead of tension), possible increase in leakage paths, containment, stator structure design (now all of your stators are attached to a stationary shaft that can only be supported at the ends), control of variable stator vanes, getting the fuel into the combustor, how do mount the engine, etc.
Edit: Just reread the question and realized you were also talking about making the combustor bigger. Modern combustors are getting smaller and are much smaller than they were in the original jet engines. You can also see in the pictures above, they already don't use all of the space available to them and if they needed more space the arm (shaft) connecting the HPT and HPC could be lowered some more.

A rotating drum will have a higher inertia. When the pilot commands a thrust increase, spooling up the drum will take longer than spooling up a shaft. Also, a big drum is harder to balance well than a thin shaft.
However, what you propose is not so far from general practice: The high pressure spool on modern engines is quite big already, but it is still on the inside of the blades. See the cut-away picture of the General Electric Passport engine for example (source):

Here it is interesting to see that the core components are quite a bit smaller than the channel for the bypass air, but the high pressure spool uses all the space left by the combustor. You can also see that the low pressure spool must be thin to allow enough cross section for the first stages of the high pressure compressor. Using a drum here would not only increase the inertia of the low pressure spool, but also of the whole high pressure compressor. Putting the drum on the outside of the blades would put it in the way of bleed air lines, accessories driveshafts and fuel lines. Also, making the stator vanes moveable would be much harder. The cut-away should make it obvious that the diameter of the high pressure section was kept as small as practical.

The OD of the spools where the arms connect the individual rotors is determined by getting the arms in the correct position to seal the stator bypass. The higher the OD on the arms the thinner you can make them, as well, since $\tau =Fr$ so as the radius increase, the force goes down, which is why the arm is so high under the combustor.

@OSUZorba: I'm glad we have an engine expert around here!

You seem to be describing an Exoskeletal Turbine design. I'm no expert, only vaguely associated with the industry. They're attractive in part because your rotor materials only need to be strong in compression, so you can use e.g. ceramics. Further, the hollow inner volume can be reused for an alternative flow-path or engine type, which opens up options for sc/ramjet options with a conventional turbofan wrapped around it. NASA's investigated it, with major findings that the bearings technology didn't exist at the time and that support equipment would likely eat up any advantage gained, for roughly equivalent performance. Materials improvements may shift the balance either way.

Welcome to [Aviation.SE]. Great information in your first answer!

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Main structural (and any other) impact of hard landing
I was reading this question related to the parts suffering the most G forces. Reflecting about that question I question myself about hard landing, which is not a normal operation.
So, my especific question is: when an airplane is suffering a hard landing event, which are the structural parts (and any other part that might not be structural and I might be forgetting) that are suffering the main damages and why?
Will this answer depend on the airplane vibration mode (i.e. location of engine masses)?

here are the main parts at risk for structural integrity after a hard landing:
the landing gear itself and the attachment points of the gear to the fuselage and/or wings
the fuselage-to-wing attachment structures
the engine-to-wing (pylon) or engine-to-fuselage attachment structures
the main fuselage structure in the immediate vicinity of the wing attachment points
If the hard landing involves a tail strike, then the integrity of the aft pressure bulkhead is at risk.
Note that the structural integrity risk is not simply one of prompt failure due to an overstress impulse load, but also subsequent premature fatigue failure arising from microcracks caused by the hard landing stresses- especially if that cracking occurs in a location which is inaccessible for inspection.

I believe it could be implied with niels nielson’s answer, but do not neglect inspection (and torque) of fasteners.
In my limited experience, all structures and gear can appear undamaged and normal but the fasteners (e.g., of the gear, engine mounts, wing attachments, etc.) may be stretched, bent, or deformed as would be evident only by torque checks and very close inspection, even removal.
Replace all suspect fasteners and kindly resist the temptation to simply retorque them. I know of no specific criteria but anything more than a 1/4 turn to retorque to meet spec would raise my eyebrows and require more inspection and/or replacement. (And of course this would be done by only those legally authorized to make the particular aircraft’s inspections and repairs.)


text:
Was the F4F-1 Wildcat ever built?
The original design for the F4F Wildcat was as a biplane, but with the -2 model, it was redesigned as a monoplane. I am wondering if the -1 model was ever actually built, or did it remain as a blueprint only?

A design was drawn up but it was never built. Grumman opted to move directly to the monoplane design instead.

According to aviation-history.com the design never left the drawing board.

In 1935, the US Navy ordered the XF4F-1 as a backup to the US Navy’s first monoplane fighter, the F2A-1 Brewster Buffalo. However, data from the Buffalo demonstrated that the biplane fighters could no longer match the performance of the monoplane. It was now apparent that biplane fighter's days were numbered as the XF4F-1 performed no better than the re-engined F3F.
The design never went further than the drawing board and the Navy’s thinking had finally reached the conclusion that the navy fighters were lagging far behind land-based fighters. The biplane design was scrapped in favor of a new prototype, designated the XF4F-2 (G-18).

This seems to suggest that it was built, at least as a prototype.


text:
Electric trainer aircraft has engine weighing 11Kg , how much would a comparable conventional engine weigh?
The Pipstrel Alpha Electro Trainer has an electric engine which weighs 11Kg.
How much would the engine of a comparable gasoline powered aircraft weigh ?

You need to compare apples and apples... What does the engine and the batteries weigh, and compare that with a gas engine and fuel.

I'm not sure why this is a question, the Wikipedia article the OP linked to has the specifications for the gas powered Pipstrel Alpha, and fuel capacity.

What I've been able to discern from the electric paramotor business is that the power density of electric power per pound overall is roughly 1/3rd that of a gasoline engine.  Good enough for lots of uses, but not quite there as a universal application.

The electric motor may weigh only 11Kg but there are other weights that should be included in comparison to a piston engine. In addition to the batteries there is a liquid cooling system for the electric motor. This cooling system includes coolant, radiator, pump, controls, etc. that are not found on a conventional air cooled piston engine.

The Pipstrel Alpha Electro is an electric version of the Pipistrel Alpha Trainer which has a Rotax 912ul engine. The Rotax 912ul weighs 60kg.
For comparisons sake;
The Pipstrel Alpha Electro has a 11kg motor and 126kg of Lipo battery cells for a total of 137kg and about 1hr of flight time per charge.
The Pipistrel Alpha trainer has a 60kg engine and carries 35.5kg of fuel for a total of 95.5kg and 5-6hrs of flight time per tank.
flight time estimates are made assuming the planes are being used in a touch and go trainer role.  At cruise speed flight times would be less.
EDIT for question in comments: The electric motor and Rotax engine both make 80hp. specs on the electric plane can be found here: https://www.aopa.org/news-and-media/all-news/2015/october/pilot/f_pipistrel
and
https://www.pipistrel-usa.com/alpha-electro/

Much more comprehensive answer than mine, but you might need some references.

Not sure if this actually answer the "comparable" part of the question, i.e. is the electric motor 80HP? If not then the 60kg gas engine shouldn't be considered "comparable", even though it's used on a similar airplane.

@user3528438 The Siemens 60-kW/80-hp electric motor and the Rotax 912ul both make 80hp so it is very much "comparable". Specs can be found here: https://www.aopa.org/news-and-media/all-news/2015/october/pilot/f_pipistrel I'll edit my answer though.

Let me begin by saying I agree with the comment left by Ron, batteries are very heavy!
But to answer your question, Most medium sized trainers use a "360" engine built by either Lycoming or Continental.
A Lycoming O-360, an older, N/A engine fitted with a carburetor, weigh between 124.7kg and 137.4kg. Source.
Whereas when you "Kit Out" your Lycoming engine with a turbo and fuel injectors (TS-IO-360), they can weigh between 171.9kg and 184.6kg. Source.
If you're interested about what all the numbers and letters in an aircraft engine code mean, have a look at this Continental PDF documenting it all.

The non-electric Pipstrel uses a Rotax 912UL, it would be better to compare it to that instead of a Lycoming or Continental.

Yeah, in retrospect I agree.
I just know very little about the Rotax engines and thought it was best to share the knowledge I have rather than tell OP about something I'm not sure about.


text:
How do I get certified to fly under FAA 333?
tl;dr: How do I go from non-airman to commercial UAS pilot under an FAA 333 exemption?
I understand that the rules are imminently changing, but waiting for the formal implementation of the NPRM could mean a very long wait before even getting started, and there is already a backlog of section 333 petitions.
Say I have a small (under 55 pounds) quadrotor aircraft, and I want to fly it for "civil operations" (commercial). As a non-airman, what do I need to do before I can operate the craft in this manner?
As near as I can determine, the process is something like this:

Obtain a pilot license
File a petition for part 333 exemption
Acquire either a SAC or Waiver/CoA

Do I really need to obtain a pilot license of some sort for real aircraft (either PPL, sport, or recreational) for this purpose, or can I legally operate the craft using some other short-cut? Is there a difference if I plan to fly a craft with a SAC rather than a craft which has none?

The FAA's FAQ says:

Is an FAA-issued pilot certificate required to operate a civil UAS under an experimental airworthiness certificate or a grant of
  exemption under Section 333?
If the aircraft is issued an
  airworthiness certificate, a pilot certificate is required.5
Pilot
  certification requirements for petitions for exemption under Section
  333 are evaluated on a case-by-case basis.

The footnote links to this FAA order called Airworthiness Certification of Unmanned Aircraft Systems and Optionally Piloted Aircraft, which says:

b. UA PIC Certification and Ratings Requirements.
(1) The PIC must hold and be in possession of, at a minimum, an FAA private pilot
  certificate, with either an airplane, rotorcraft, or powered-lift
  category; with single- or multiengine class ratings, appropriate to
  the type of UA being operated.
(2) The PIC must have and be in
  possession of a valid second-class (or higher) airman medical
  certificate issued under 14 CFR part 67, Medical Standards and
  Certification.

So, it looks like there are two possibilities:

Your UAS has an airworthiness certificate, therefore you need a pilot's license per the Order (private at least, not sport or recreational)
Your UAS doesn't have an airworthiness certificate, therefore your section 333 petition will be evaluated individually and the FAA will tell you if you need a license or not

Since the OP asked for "commercial UAS pilot", wouldn't the minimum license be commercial rather than private?

@DanPichelman No, the order says private is good enough. But interestingly, it does say you need a second-class medical (which is required for commercial pilots). Private pilots only need a third-class one.

Go figure.  Thanks


text:
Can passengers boarding back first tip back an airplane?
I came across this image and don't have any details nor do I know the exact reason, but it made me ask myself if passengers boarding to the back first can tip back an airplane?

Possible duplicate: https://aviation.stackexchange.com/q/15042/7532

Related: https://aviation.stackexchange.com/q/1659/7532

Also: https://aviation.stackexchange.com/q/9975/7532

The photo is from a 2021 Sept 18 news report: "United flight 2509 flying from Los Angeles, California to Lewiston, Idaho landed without incident. Due to a shift in weight and balance during the offloading process, the tail of the aircraft tipped backward.  No injuries were reported among our customers, crew or ground personnel. The return flight will depart on a different aircraft as originally planned."

Not if everything else is normal. Airliners are built with the expectation that they will be boarded to the back first on some airlines.

The CG limits for the 737 are approximately 12%-31% MAC, depending on model, and that is at MTOW. At less than MTOW, the CG can be as far back as 36% MAC. You'd have to put more passengers in the tail end than it's able to normally accommodate - think clown car packing density.
To be specific, taking 173 pax at 200 lbs each, at near-MTOW, having only the tail 20% seated would shift the CG 1730.240%*200/173000 = 1.6% back. The plane shouldn't be just 1.6% away from the limit in the first place. And this assumes a straight tube without an actual tail and aft pressure bulkhead.
To tip the airplane back, it needs to also have very wrong, tail-heavy loading in the cargo/luggage compartment.


text:
What are the least and most turbulent hours of the day?
Yes, it is quieter on the early morning, but what about the other hours of the 24 hours of the day?
I want to know statistically which hours of the day are the most and least turbulent, is there any data?
PS: This question excludes wake turbulence type.

Well It depends on length of day but basically you are going to be looking at mid-afternoon as the most turbulent.  But you are asking about data so this isn't really an answer.

At low levels, convective turbulence is at its lowest early in the morning and late in the afternoon/evening when the Sun's heat is weak.  Mid-afternoon, when the surface has heated-up ("two o 'clock thermals") is worst.  But there's also mechanical turbulence from winds converging or passing over terrain, they don't usually depend on the time of day.


text:
What is the typical cruising altitude for a civil airplane?
I'd like to know the interval of altitudes airliners usually cruise at (and its reasons)? And if there are international restrictions about this?
Also I read somewhere that recently about a restriction that airliners should not fly bellow 26000 ft over egypt. How hard that restriction would be?

@mins of course but I presume that the restriction does not apply to the airport surrounding area

Yes there are recommendations over some hostile or unsafe regions. For Egypt there is a FAA NOTAM (FDC 4/8353) since a long time and EU has joined the warning too. See: Operators Warned to Avoid Flying Over Egypt’s Sinai Peninsula and in general International Overflight of Hostile Areas and FAA restrictions.

@mins yes in your link it mentioned that FL260 limit is sort of a recommendation

I think there are already plenty of answers here to give you a clue:

How do pilots decide what their cruising altitude will be?
why are regional cruising altitudes higher than those of international much longer flights?
How can you find the best altitude for range or speed?
Why are many jet aircraft designed to cruise around FL350-370?

Normally, atitude restrictions give a top limit. A minimum altitude is hard to enforce when one engine fails - then the aircraft can only continue at a lower altitude (typically around 15,000 ft for a two-engined jet) or the cabin pressurization fails (now the aircraft needs to continue below 10,000 ft).
This is speculation, but maybe the 26,000 ft limit is caused by information that rebels in the West Sinai have gotten hold of shoulder-launched anti-aircraft missiles. Those have a fairly low upper limit to their effectiveness.

thank you and what would be typical top limit?

There is no "typical" top limit, and ATC stops at 50,000 ft, well above regular airliner capabilities. You have en-route top limits normally for individual airplanes in case of system failure (where they are flightworthy up to that altitude) or to keep departing and arriving traffic apart near an airport. Also, some restricted airspace (say, for military exercises) can be overflown only above a certain altitude.

@PeterKämpf the ceiling for ATC isn't constant. over the US, class G starts at 60,000 ft.

@casey above FL600 is class E.

It depends on the type of airliner.  They all fly high to take advantage of a higher "true airspeed" (TAS).
Smaller turboprops like the Beech 1900, SA226, and SA227 might cruise in the low to mid 20,000 foot band.  Jets will usually cruise above 30,000 feet.
The air is thinner at high altitudes which reduces drag on the airplane and allows it to slip through the air at a higher "true" airspeed.  Turbine engines are optimized to run at high altitudes and by running in to 20,000 - 30,000 foot altitude band, they can still generate enough power to propel the aircraft along at a couple hundred knots using far less fuel than they would at sea level.
The airspeed indicator in a jet cockpit will indicate something like 250 knots, the true airspeed will actually be 400 knots, and the ground speed will equal true airspeed plus or minus the wind component.  Why the big difference between indicated airspeed and true airspeed?  The thin air "tricks" the airspeed indicator!  There are simply fewer air molecules at 20,000 than at seal level and since the airspeed indicator in the cockpit needs air molecules to determine the speed, the thinner air fools the indicator into thinking the plane is going slower than it really is.
The true airspeed corrects for this by taking into account the thinner air and temperature to tell the pilot "hey, your airspeed indicator tells you that you're speed is 250 knots.  But since you're up high and the air is thinner, I've done the calculation to determine your REAL speed through the air...it's 400 knots!"
So...
At sea level, a jet will indicate 250 knots.  The atmosphere is thick at sea level so the indicated and true airspeed will be fairly close.
The same jet at 30,000 feet will indicate 250 knots but the true airspeed will be much higher.
Taken to an extreme, a spacecraft in orbit has a speed of many thousands of miles per hour.  What's the airspeed?  ZERO!  There is no more atmosphere so the airspeed indicator (which relies on air molecules, remember) can't register an airspeed.  The ground speed of the spacecraft is extremely high, though.  Now, toss in a thin atmosphere and your indicator will begin to show you a speed.  It will be low because the air is thin but your actual (TRUE) airspeed will be very high.
As far as the Egyptian altitude restrictions go, I haven't a clue.


text:
What types of attitude indicators are there?
Learning to fly in general aviation in the west since the mid 2000s I'm used to the common attitude indicators like this one.

I've also seen older ones that are black and white as well as many varieties of attitude indicator display arrangements in glass panels.
My question deals with other ways of displaying this information. I've heard pilots talk about how some eastern airplanes have very different ways of presenting pilots with attitude information. What approaches are out there? What do they look like and where are they found?

Russian-made aircraft have attitude indicators that work exactly opposite of western ones.

source
with the western style, on the left, the virtual airplane figure is fixed so it's always upright to the pilot and the horizon moves. On the Russian style AI the horizon is fixed so it is level from the pilots POV and the aircraft symbol moves.
Just two ways of looking at it, but if you're used to one the other can be confusing.

It's going to take some time to get my head wrapped around that Russian one. It looks like the plane is almost on knife-edge!

@FreeMan Took me a minute too. It helps if you top your head to the right so the ia's look level to you. Then you can see the horizon is flat and the plane icon is tilted. Can you imagine somebody who flies both?!

Weird. It seems like the Russian one shown here completely defeats the purpose of using an artificial horizon. I thought the whole point of having an artificial horizon was that it mimics what you would see from the actual horizon if you were looking out the window at it.

If you have a Russian plane and you want to use the Western indicator, can you just remove one and plug in another? Or are they not interchangeable?

@FreeMan That is because the picture depicted the instruments tilted too, which is very confusing. When you are flying the plane you do not perceive that: basically you have to rotate the instruments by 40° to the left and that's what you would see. Both the styles are quite understandable, but if you confuse them, you crash the plane (see for instance https://en.wikipedia.org/wiki/Aeroflot_Flight_821).

I see things a step further than it just being "two ways of looking at it." When I use a western attitude indicator I actually visualize the horizon on the AI extending out in all directions. I use this as a mental process for augmenting my situational awareness. This isn't possible on the Russian AI.

There's also the 3D attitude indicator from old fighter planes.

So this is a compass, pitch and bank indicator? I can see that getting confusing when you're in a 90 degree bank trying to avoid the mean end of a Messerschmitt!

Oh, just call it a navball!

No Messerschmitt  would be chasing you if you were looking at this attitude indicator, but maybe a MiG

Does this behave like "Western attitude indicator" nowadays? It looks like the plane symbol is always static and the background moves behind it.

@MikkoRantalainen, yes, the ball moves. The white W wings are fixed. The yellow bars move up/down and left/right to guide pilot, they can be controlled by a variety of navigation or targeting inputs. There is a glide slope needle on the left side. The knob adjusts the pitch angle. This instrument is showing the nose is high, wings fairly level, and heading 028..


text:
How do I know if I'm able to use LPV minimums on a GPS approach?
Many GPS approaches have lower descent minimums labeled LPV. How do I know if my aircraft is equipped to allow me to fly to those LPV minimums? And while we're at it, it would be good to know which minimums to use from the whole array of minimums found on GPS approaches.

Related questions here and here

There are many websites / blog pages that compare and contrast the different approaches. All you need to do is google a bit.
That said, if your plane can fly LPV approaches you'd know that. It requires WAAS-quality GPS receiver, and to install that requires at least a logbook entry.
Note that it's not only a matter of equipment. Even when the plane is equipped to fly LPV approaches, there can be certain conditions when you can't use LPV minimums (satellites, etc.) If you CAN use LPV minimums, the GPS unit MUST show you "LPV" in the annunciator window. It's possible that you're LPV-equipped, but your annunciator window shows you only "LNAV", for example.
So, check your documents, talk to your mechanic, you wouldn't really NOT know if your plane was ever upgraded for LPV approaches. If it's your own, you'd know and have discussed it and most likely given the mechanic the task to upgrade it. If it's a rental, or you're just buying it, look at the logs and ask the seller. And keep in mind, the annunciator window will show you the lowest GPS approach type that it senses is safe. Usually on an LPV-equipped plane that will be "LPV", but it's entirely possible it will only show "LNAV", or something else. For you to use LPV minimums also depends on the actual flight condition.

The previous answer nailed it on the regulatory/technical capacity to fly LPV, but then we have the operational capacity.
LPV approaches are the most stringent for SBAS/WAAS usage. They depend not only on having enough GPS satellites in view but also that those satellites have the top accuracy status (which is dynamic and depends on how many ground stations are seeing the satellite and if the satellite is operating in perfect health).
Its possible to do a predictive WAAS status prediction to find out if LPV accuracy should be available at your estimated time of arrival. Meaning if the GPS satellite constellation is expect to be good, excluding satellites which are bad or planned maintenance outages. But this doesn't assure something unexpected won't happen by the time you actually fly the approach.
LPVs can also be downrated from their published minima to 250DH minima if the geometry is good enough for 250DH but not for the actual DH (assuming of course the DH is lower than 250ft).
I suggest looking at the http://www.nstb.tc.faa.gov site. It's an official FAA site that has a realtime LPV200/LPV/VNAV plot (http://www.nstb.tc.faa.gov/RT_VerticalProtectionLevel.htm). This is what's going on right now. LPV200 is the full LPV down to 200DH if a approach allows (inside the yellow contour line). LPV is the downrated 250ft DH LPV (between the yellow and red contours).
We also have the daily (past) plot of:
http://www.nstb.tc.faa.gov/24Hr_WaasLPV200.htm - LPV200
http://www.nstb.tc.faa.gov/24Hr_WaasLPV.htm - LPV250
http://www.nstb.tc.faa.gov/24Hr_WaasLP.htm - LP
LPV is highly dependent on the iono correction grid. If there's a severe problem in the ionosphere, LPV200 and 250 could be disrupted. This is very rare and more likely up in Alaska.
For instance its normal to have outages in the California cost for a few minutes nearly every day. Its the result of instants when there are enough satellites in view, but the reference stations don't have enough satellites to properly calculate the iono grid for the nearby ocean (iono grids are used for the region where the signal travels through the ionosphere rather than for your ground location, which changes quite a bit for satellites closer to the horizon or even on a 45 degree angle).
In the future dual frequency and multi constellation SBAS will pretty much put this to end permanently. Instead of using only US GPS, Europe Galileo, Russian Glonass and others will be allowed, as well as having the receivers do the iono calculations by themselves using L1+L5 (GPS/Galileo) and L1+L3 (Glonass). This will require new receivers for most GA aircraft but is not expected to be mandated since LP/LPV already works extremely well. But it would enable LPV/LP in Hawaii, 100% of Alaska, as well as all of Central/South America.


text:
How is the steepness of turn bank properly defined?
In aviation we often refer to the bank of a turn as shallow, medium  or steep. I remember reading somewhere that the definition is not universal as to how many degrees it is, but it has to do with the airplane and whether the bank tends to roll back to level, remain where it is or continue to steepen. Does anyone know if this is a correct definition or have any references to share?

"the bank tends to roll back to level, remain where it is or continue to steepen. " That sounds more like a stability definition. There is neutral, positive, and negative stability. Is it at all possible you are mixing up steepness with stability? They both start with "st" :)

No I'm not mixing anything up. Read the accepted answer.

OK, all right then!

I don't know if this is only definition out there, but according to the FAA's Airplane Flying Handbook:

Shallow turns are those in which the bank (less than approximately
  20°) is so shallow that the inherent lateral stability of the airplane
  is acting to level the wings unless some aileron is applied to
  maintain the bank.
Medium turns are those resulting from a degree of bank (approximately
  20° to 45°) at which the air- plane remains at a constant bank.
Steep turns are those resulting from a degree of bank (45° or more) at
  which the “overbanking tendency” of an airplane overcomes stability,
  and the bank increases unless aileron is applied to prevent it.

Also very closely related is the turning radius, which also depends on the speed. For example, a Rate 1 turn is defined as a turn that changes the aircraft heading 3 degrees per second, so it takes 2 minutes (120 seconds) to turn a full 360 degree circle.
If you are in an ultralight, doing 50 knots, you need approx. 50 / 10 = 5 + 7, or 12 degrees of bank to achieve a full circle in 2 minutes.
If you are in a 747 flying at 180 knots, you need 180 /10 = 18 + 7, or 25 degrees of bank to achieve a full circle in 2 minutes.
Both turns might be classed as "shallow" (although on that definition I would hate to do a steep turn in a 747).

Radius and  bank angle are related only if you include speed, which is not in the original question.

Oops! The point I was making is that there are many different definitions. Also what might be considered steep in one aircraft type might not be in another. The UK CAA tend to have three bank angles that interest them: 15 degrees, 30 degrees and 60 degrees. 15 being related to slow flight, 30 the max. for turns in the circuit and 60 the max. for normal category aircraft although they mention these angles many times in their docs.


text:
What is the airspeed velocity of an unladen swallow?
I'm considering the airspeed velocity of an unladen swallow because the swallow may present an opportunity for the observation of nature to lead to better design in aviation. Many technologies built by humans are inspired by designs in nature.
If one were to set out to determine the airspeed velocity of an unladen swallow, how might they go about this?

This question does not appear to be about aviation. It might be suitable for http://biology.stackexchange.com.

Further, use of the absurd term “airspeed velocity” makes this sound like a joke rather than serious question (speed and velocity are near-synonyms, with ‘speed’ usually used for the scalar and ‘velocity’ for the vector quantity).

What do you mean by measuring 'airspeed velocity' of 'unladen' swallow? Speed is a scalar and velocity is a vector; I don't think anyone uses different methods for measuring speeds of laden and unladen birds.

@JanHudec makes this sound like a joke I have to infer then that you have never heard of Monty Python

@Federico, of course I did. That's just more reason to think it is a joke (along with similarly silly “unladen swallow” as if swallows usually carried burdens).

I'm voting to close this question as off-topic because it's a joke.

This guy did put some serious though into it though. http://style.org/unladenswallow/

For curiosity: What is a laden or unladen bird, my bilingual dictionary isn't helpful in this context :(

These two videos should provide some context.
http://youtu.be/liIlW-ovx0Y
http://youtu.be/_7iXw9zZrLo

@mins lade is a verb and a synonym of load. So laden means carrying a load or burden.

You guys are killing me with the down votes! This is such a good question. Haters.

If this weren't off-topic, we would still need to know your favorite color and the capital of Assyria before we could proceed with a useful answer.

Wait. What? I don't know that. Ahhhhhh!!!

@RyanBurnette: Thank you for such enlightenment. The question should be migrated. Thanks TomMcW too.

@RyanBurnette if you hadn't already asked this I was going to.

In general, the method of getting the airspeed of any bird is pretty much the same:

Measure the ground speed of the bird.
Measure the wind speed in the area.
Calculate the airspeed of the bird from these two values.

The following excerpt from Air speeds of migrating birds observed by ornithodolite and compared with predictions from flight theory by C. J. Pennycuick et al. published by the Royal Society details one such method:

We measured the bird's air speed, for comparison with the predictions of theory, in two stages. First, we measured the ground speed vector, consisting of the bird's speed relative to the observer's position on the shore and its track direction, i.e. the direction in which the bird was moving over the ground, measured clockwise from true north. We also measured the wind vector consisting of the wind speed and the direction from which the wind was blowing, and then obtained the air speed vector (air speed and heading direction) by vector subtraction of the wind vector from the ground speed vector

Other researchers have used  Doppler radar for the same purpose and in case of high flying birds, one can simply fly along with the birds at the same speed and measure the airspeed.

You didn't ask if we're talking African or European.

Assumptions must be made because your question is incomplete.  Be more specific next time.


text:
What is this oddly-shaped plane?
I saw this plane yesterday:

What is this plane? And what is the reason for it's odd-looking nose?

Related: How do the Airbus Beluga's wing/control surfaces deal with its high center of gravity?

That's any easy one. Its to carry large amounts of cargo. Air Beluga generally carries parts of aircraft around the world look at aeroalias answer to see what I mean when it carries aircraft parts.

@Ethan Specifically, large volume but relatively low-weight cargo. There's no airline called "Air Beluga": the model of plane is called the Beluga and they're owned by Airbus. Also, they don't usually fly "around the world" but between a few Airbus factories within Europe. They do occasionally get hired out to other companies to carry other cargoes to other places but that's not their main use.

@Ethan Also, there's no need to post a comment saying "Look at so-and-so's answer" -- especially when it's the only answer! People who are looking at the question are going to look at the answers anyway, if they're interested, and you can encourage them to read the best one first by upvoting it.

It is the Airbus A300-600ST (Super Transporter) Beluga, which is a version of the A300 wide-body airliner modified to carry aircraft parts and oversized cargo.

Source: berlin-aviation-forum.com
The nose is shaped like that as the upper portion is basically a cargo hold and is used to transport oversize cargo, especially aircraft parts.

"F-GSTB - 2 Airbus A.300B4-608ST Beluga Airbus (8634655670)" by aeroprints.com. Licensed under CC BY-SA 3.0 via Commons.
The cockpit was lowered below the cargo deck so that the cargo area could be loaded without having to disconnect any of the vital electric and hydraulic lines running to the rest of the plane. However the A300’s lower fuselage, wings and landing gears were retained.
The following image shows the changes that were made in the nose section from A300 to the Beluga.

Source: allaboutguppys.com

First to say "planeception" wins a prize.  Yay, it's me!  What do I win?

@DavidRicherby You win a "yo dawg!" ;)

@Michael: This one?.

Also useful for carrying elephants.

But, where do you carry unassembled Beluga parts?

@Dronz personal experience? :D

@digitgopher I wish! I do know that getting elephants around comfortably is an issue, and these days, zoos are often retiring their elephants to better pastures.


text:
Double deck vs wide body airliner, why would anyone build a double deck one?
I don't understand why double deck aircraft (or multiple decks in general) are a good idea. They must be, because there is a significant amount of them.
But given a cross area, which should roughly correlate to the drag of the body, wouldn't a wider cross area provide some additional lift for "free" as compared to having higher lift wings instead?
Why don't we see very wide airliners where the body provides a siginificant amount of lift?

That WP list can be pretty much distilled down to two recent planes - 747 and A380  That isn't a "significant number" by any counting method.

@MikeBrockington A quick glance at the Wikipedia pages of those planes tells you that airlines spent hundreds of billions USD on those two planes, over a period of 50 years. That's clearly significant.

My point was only two designs out of many, but even on spend, this is a tiny fraction of overall spend. For example, (according to www.stastista.com) "Airbus Group's global revenue amounted to approximately 64 billion euros in the fiscal year of 2018."

@MikeBrockington That would be 2 design families, not just 2 designs, correct? There's the 747-100, 747-200, 747-300, etc.

@Mast  Does that really alter the proportions? After all, the A380 didn't end up being a very big family, but most other basic designs end up with multiple models over time - the 737 alone probably has as many different variants as the 747 (even if some of them aren't flying just now!).

Seems they may be coming...

That link reminded me how much I dislike the fact that Wikipedia doesn't detect when you aren't running a mobile browser and give you the desktop page instead.

Problems with just making the fuselages wider:
With the wing or main gear supporting the center of the fuselage, the front and back are cantilevered. Think of a hard landing or 2.5G pull up. The most effective structure to support bending is tall, with material on top and bottom (like an I-beam). A flat fuselage would have to be heavier to provide the same strength.
Depending on how you shape it, you're trading wasted space above and below the cabin for having an efficient pressure vessel shape. If you want to fit standard cargo containers below, that constrains your options.
Although you may get some amount of lift from the fuselage it will probably still push the wings and engines outwards by some amount, creating issues with gate width limitations.
In order to meet evacuation requirements, there would need to be more doors or at least emergency exits for a given length. That could mean more cross aisles and less room for seats. It may also be easier to access multiple doors for passenger to board/disembark if they are on different levels than the same level.
Lower ratio of windows to seats. Less natural light. People like window seats.

Re gate width limitations, you'd have to make folding wingtips, like the newer version of the Boeing 777 (or whatever number).

I think a number of your objections would be solved with a delta wing design.

@nick012000: But I think delta wings are pretty inefficient at subsonic speeds, no?

you cant expect the rump to add significant aerodynamic lift - except for delta shapes that become increasingly unstable with growing size
Add to that the requirements for air safety. emergency deboarding gets more difficult with one flat huge room, compared to multiple decks which allow for more doors

Apart from the bending, doesn't a circle give the optimal area to circumference, i.e. passenger volume to wall ratio?

@JollyJoker: Yes, if your cargo is liquid that can fill the entire volume without wasted space in curved areas, unlike passengers who enjoy flat floors.  (Luggage and other cargo are probably also in squarish form factors.)

@PeterCordes At some point the circle becomes large enough that you can fit two decks though

@JollyJoker: I was talking about the problem of low "ceiling" near the walls making the potential floor space under it maybe unusable for a seat in the upper half.  Slightly oval makes the wall closer to vertical.  Or maybe that's not a problem and overhead bins fill up that curved space anyway.

@PeterCordes https://i.sstatic.net/t6jxm.jpg They do seem to be slightly oval in practice

@PeterCordes I am pretty sure United, at least, is trying to figure out how to turn passengers into some sort of liquid or amorphous blob

Well, you only need so much vertical height between decks, say 7 or 8 ft, so once the tube diameter gets so big, you end up with simply too much wasted overhead space.  Might as well jam some people up there, which means putting a second deck in.

But why tubes? The large commercial double deckers I've seen aren't tubes, they are ellipses with the longer axis vertical.

You have a circle, and the top of the circle would mostly go to waste but isn't quite tall enough, and you can't make it wider, so you ovalize it a bit to make it a little taller.

@JohnK: That sounds exactly the wrong way around. The center is already the highest part, it's tall enough. It's indeed going to waste, there's only so much space you can sensibly use for overhead bins. So why make it even higher? The problematic areas are the top level windows, which are going to be angled inwards.

@MSalters Might it be an aerodynamic handling problem that encourages planes to have a slightly taller than wide body? Perhaps a mechanical reason (attaching the wings comes to mind).

@Mast: From an engineering viewpoint, the wings are what keeps the airplane up. The fuselage is what's attached. And that's probably the real reason: the fuselage is subject to bending. It's only supported in the middle. The internal pressure may cause a symmetric force on the hull, but the bending forces concentrate on the top and bottom.

The fuselage is under pressure at altitude, and acts as a pressure vessel. The lightest pressure vessel we can build has a circular cross section. The wide body you mention sounds like a blended body shape, which may add lift but also weight as this answer mentions.
We only need a certain height to walk around in a fuselage. Build a larger aeroplane with a circular cross section, and at a certain point we can fit twice the walk-around height on top of each other.

Picture above is from Torenbeek, Synthesis of Subsonic Airplane Design, and shows the cross sections considered by Douglas for what ended up to be the DC-10. All connected cross-sections are circular, with a straight panel (floor, wall) at the connection points to absorb tension.

Elliptical cross sections are very sub-optimal - the section with larger radius wants to expand more and needs to be stiffened by a wider rib section, ending up with an almost circular cross section at the expense of much added weight.

I'm curious - how much of a consideration is pressure in fuselage design? The pressure differential is pretty much capped at 1atm, right? That's only 14psi which is below the level that would require ASME-certification.

@Dancrumb  It’s not only pressure that is a consideration, weight is as well...

@Dancrumb 14 psi is a bit more than one ton per square foot. In practice, the difference between cabin altitude (Let's say, 8000ft) and max cruising altitude (maybe 42,000ft) is going to be more like 8.5psi. Bit more than 1200lbs/sq ft. Not trivial.

@Solomon Slow It's a bit less than a proper ton per square foot.

@BlokeDownThePub, I thought that was spelled, "tonne." Or, maybe you were speaking of the twenty hundredweight kind of ton.

The fuselage does not provide a useful amount of lift. Wings are far more efficient at this. You need high aspect ratio for efficient lift generation - a fuselage is much less than 1.
The best structural efficiency is provided by a fully circular fuselage, or by one that is a bit taller than it is wider - a contest between lower hoop stresses and a taller beam. A wider fuselage has both more hoop stresses and more longitudinal stresses.
About half the airliners are non-circular, but they're all taller than they're wider.

There are several factors that determine the cross section of a passenger aircraft; the most obvious one is that absolutely nothing is for free - there can be no unjustified weight/surface area/volume/complexity/uncertainty in aircraft design - everything is counted, everything is assigned a priority.
Also obvious is that passengers are higher than they are wide, for logistical purposes.
Then there is that a pressurised vessel composed of the thinnest, lightest, cheapest possible material wants to assume a circular shape.
Less obvious is that passenger aircraft are not built on a keel, or ladder frame, or chassis, or assembled from self-contained modules. The rib & stringer frames you see in aircraft factories are quite fragile & vulnerable until they are skinned - even then they have little strength until final assembly. Fuselages are more like an inflated condom made from postage stamps, rather than a short section of sturdy water pipe. The fuselage has to be many things, bridge, tunnel, pressure vessel, dangerous goods storage, bus, storm shelter, gasworks, auditorium, and always make more money than it costs.
To save weight, & solve as many other engineering problems as possible, the fuselage & floor are designed as a tension structure. Any glance at a cross-sectional diagram will illustrate this, it's easy to imagine a cable holding in the sides instead of floor beams. Of course the floor has to carry vertical loads of passengers & equipment, but that's separate to its contribution to the entire structure.
There really aren't that many double deck aircraft; 747s have a quarterdeck area behind the cockpit that has seating, but that's more a historic quirk, rather than the reason for its success; it was designed as a bar/lounge area. Turning it into limited seating worked because it was, well, limited. The A380 has been a gallant failure,there will be less flying every month. .
A second deck on an aircraft just can't create more problems than it solves. Even when the problems are external to the aircraft - terminals, financing, politics, etc.
If you wanted to have a fuselage wider than it is high, the floor would need to become a heavier compression structure (as pressurisation tries to circularise the fuselage), now subject to bending forces that need to be braced against, adding to structure, adding to weight, adding to cost, depriving space from revenue-earning volume, adding to costs, and so on.
And that's just the floor. You've now changed the shape of the tube you're using to join the tail to the wings & the wings to the nose. You've massively increased the strength of the fuselage in yaw (the least important parameter) while weakening it in pitch (really important), plus probably introducing new, very novel, twisting tendencies into your combination tension/compression structure.
This can be ameliorated by having two circular cross-sections next to each other, instead of a wide ellipse, with the circles overlapping & a vertical wall, or series of posts, or tension members of some sort, running down the length of the cabin.
Part of your sales pitch to the airlines for a very wide cabin was that they could vary whether it was 13 across or 22 across or a flying tapas bar or whatever they desired. Except that now, for the first time, they have an aircraft with a physical divider down the middle.
The regulators also frown on these dividers, because they have tested evacuations from the new triple aisle/quadruple aisle airliner & found that they turn into soccer riots unless entire rows of seats are removed, aisles are doubled in width, emergency exits are doubled in size & number with larger empty spaces around them, supervising cabin crew are increased.
The airlines will not, ever, want an aircraft like that. They want exactly what they have now, but cheaper.
So, we end up with single aisle, circular fuselages and twin aisle, circular fuselages.

The 747 was originally designed as a freighter; the upper deck provided a place to put the cockpit that wouldn't interfere with a nose door, and the passenger area was simply the streamlined fairing that blended the back of the cockpit with the main fuselage.

Because it’s only economical for long-range cargo aircraft.
There’s an interview with a Boeing executive discussing the problem with a delta wing airliner here:
https://leehamnews.com/2018/04/03/dont-look-for-commercial-bwb-airplane-any-time-soon-says-boeings-future-airplanes-head/
Quoting his statements in that interview:

“A BWB works great because it’s got [wing] span. It’s got more empty weight than a tube-and-wing configuration for a revenue ton of cargo. It’s going to carry more operational empty weight. It’s got span and it’s got a fair amount of wetted area.
Sinnett said designers will make the weight trade for span when there is a lot of range needed. For short missions, for example, four hours, the weight trade isn’t worth it.
“A four-hour mission isn’t going to work because you’ve got more weight and you’re not taking advantage of that span,” he said. Long cruise segments are required to make the aerodynamics work to their greatest benefit.

Then, a bit later:

“And, in general, unless it’s more than a 20% improvement over what you have today,” a new airplane isn’t justified. “For more than a 20% improvement, people will make the change,” he said.
Sinnett that that Boeing’s studies have only shown a few cases in which the economics are better than 20%.

Regarding passenger airliners, he says:

In a BWB, the center wing box is also where the passenger cabin is. The wing box needs to be tall enough to accommodate 95% of the world’s men to be able to stand upright.
Space is required for the carry-on bags. There has to be room above and below the floor for ventilation and systems.
“That means you have to have a certain height. Once you have a certain height, you’ve got to have a certain width for the box to work,” he said. “That translates into a certain span. You’ve got a really big airplane at that point.”
This results in the multiple hundreds of passengers, he said.

2018 Boeing says no, but 2020 Airbus says yes.

@J... Well, we'll see if they pan out into anything more than experimental unmanned drones and computer graphics models, I guess. It looks like they're at about the same point that Boeing got to before calling it off as uneconomical.

The second deck on the 747 was really solving another problem entirely. There's a good summary of the 747 in this article (and other places, if you search), but what it comes down to is that Boeing built the 747 to also be a cargo plane because they didn't think it would survive as only a passenger jet.
But that introduced a challenge: if you want to ship something that is about the same size as the inside of the aircraft, how do you actually maneuver that thing into the plane? For example, if they had a ramp in the rear, not only will the ramp make you lose floor space, but the size of the ramp and the size of the opening limits the size of what you can fit inside.
Moving the cockpit to a second deck and using a flip-up nose allows you to roll in your cargo straight forward and lets you get something inside that takes up as close to 100% of the available space as possible. For example, this video shows something being loaded into a 747 that would never fit via a ramp or a side door.
I can't find it now (maybe it was in a YouTube video), but I read that it was the 747's first customer, Pan Am, who came up with the idea of using the second deck as first class. Boeing didn't design the second deck with that in mind. It was strictly a decision for cargo.
The A380 was a response to the 747. The Wikipedia article mentions the decision for the second deck there:

The A3XX design converged on a double-decker layout that provided more passenger volume than a traditional single-deck design.

There are references after that sentence if you want to look at that. But I imagine the fact that the second deck on the 747 is "cool" had something to do with it too. But that's just me.

Cabin pressure imposes enormous forces on the body of an aircraft. The most material efficient and therefore light weight structure to handle it will be a cylinder (circular cross-section). However, when you add a deck, it can be used to take up some of the tensile load, making other cross-sections possible which become taller and narrower. They might be preferable because of the way the internal volume can be used.


text:
What section in the Federal Aviation Regulations deals with fueling commercial aircraft?
What section of the FAR deals with fueling commercial aircraft?
I can't seem to find the section. Is is under ground handling?
The actual process and safety, not the airworthiness of the fuel systems.
Thanks

Why would this be in the FARs?  It would most likely be in the AFM for the aircraft under an addendum called Fuel Servicing perhaps.

"The actual process and safety" is still pretty broad and vague. Can you elaborate on what exactly you want to know? Whatever it is, as @wbeard52 stated, it's most likely not covered in 14 CFR ("FARs") (or the CFR at all).

TypeIA yeah. specifically looking for if passengers and the APU can be running during refueling on a commercial airplane. I am an AME (A&P mechanic) and always our practises are passengers off the plane. But I cannot find any references. only refernce I can find is an AC 00-34A not to energize plane circuits during refueling. besides the necessay ones for refueling

FAR's don't address that, the only FAR that even speaks to that just says the airport must have standards. No details. For what you are asking, there is an advisory circular on the subject, AC 150/5230-4C, which provides guidance on methodology.


text:
Is there a minimum level of lighting required for a runway to have an instrument approach?
Is there a minimum amount or type of lighting, in the form of runway or approach lights, that must be present for a runway to have an instrument approach? I'm curious to know in the context of both the FAA and ICAO.

Are you asking about approaches provided to the public, for example FAA §97 approaches, or any approach, including private party approaches?

I have no idea about ICAO but as far as I can tell from the FAA's TERPS, there's no general requirement to have any runway or approach lighting at all (see below for an example), although some specific approach types (like CAT II/III ILS) do require it. Where there is lighting, it's used to reduce the visibility minimums, but the calculations assume no approach lighting by default.
This is from 3-1-2(c) (emphasis mine):

Approach lighting systems extend visual cues to the approaching pilot
  and make the runway environment apparent with less visibility than
  when such lighting is not available. For this reason, lower
  straight-in (not applicable to circling) visibility minimums may be
  established when standard or equivalent approach lighting systems are
  present.

The TERPS concept seems to be that the approach minimums can be reduced depending on the lighting type available. Section 3-3-2 has an example of calculating visibility minimums when there are no approach lights available.
As for a specific example, MDO/PAMD (Middleton Island, AK) has three approaches (2 RNAV (GPS), 1 VOR) but - as far as I can tell - no lighting of any kind. The chart supplement says:

Rwy 02–20 marked with rwy cones. Rwy 13–31 marked with cones and OTS
  lgts. Many missing.

As you might expect, the approaches are not authorized at night :-)
Finally, part 121/135 passenger flights can't land at night unless the runway is lit (121.590, 135.229), so any airport that handles those flights will have runway lighting at least, and almost certainly approach lighting too.

This is a pretty complicated question (mostly because it involves the 312 page FAA Airport Lighting Design AC 150/5340-30H).
For example in regard to edge lights, from page 3:

2.1.1. Selection Criteria
  The selection of a particular edge lighting system is generally based on the operational needs per the following guidelines:
     LIRL - install on visual runways (for runways at small airports),
     MIRL - install on visual runways or non-precision instrument runways,
     HIRL - install on precision instrument runways,
     MITL - install on taxiways and aprons at airports where runway lighting systems are installed.
  As stated, the above are general selection criteria. However, the airport surface requirements for specific approach procedures are the determining factor for system selection. See AC 150/5300-13, Airport Design, for more guidance.

It goes on to describe the different types of lighting required for MIRL/HIRL. It really goes into detail about when centerline lights, guard lights, stop bars, clearance bars, edge lights, threshold lights, etc are required for different types of runways. It goes into quite a bit of detail about what type of light is required and when.
It is important to note that these are Advisory Circulars, not FAR's. Airports really don't need to be anything more than a mowed lawn. The FAA has an airport certification process that uses these AC's as guidance for "good airport design", but the design may be modified to fit certain situations. These are only to be used as guidance in the design/construction of an airport.


text:
Why is the MD-80 magnetic compass behind the first officer's seat?
I was watching this video about the MD-83 magnetic compass and I found it interesting that it's designed the way it is. I'm not sure which of the airplanes in this series featured this strange arrangement, but it was the case on at least the MD-80 and MD-83.
The magnetic compass on these machines was located behind the first officer's seat and a series of mirrors were used to make it visible to the pilots.
Why was it built this way? Were there any advantages to it? Did they go back to a more conventional design in later models?

Pure guesswork, since I don't know.  The standby compass is positioned away from sources of interference.  Electrical generators and components, rotating components, the airframe itself etc.  It might be that on this particular design, a "normal" position could not be found with an acceptable level of interference.

See also: The Strange and Difficult to Find MD80 Standby Compass

I would agree with @Simon that the positioning is most likely related to magnetic interference (though I don't think generators would be a part of that interference). The mag compass can be hard enough to read when it is right in front of your nose; I can't imagine it is any easier when it is optically 6-8 ft away from you.

In most aircraft, the standby magnetic compass is mounted well above the main instrument panel, usually above the central window post, in order to reduce interference. In the three piece window of the MD-80 (without a central window post), this is not possible, resulting in the different location for compass.

All the Douglas jet aircraft has this type of mounting. According to this thread, the compass was first mounted on the instrument panel, which resulted in severe interference, resulting in its relocation. From the thread:

I understood that, early in the 60's, the compass was relocated in all DC-8 production due the three section type of the windshield (the compass has a lot of error/deviation at the center of the glareshield panel).

Note: I'm not able to find original documents supporting this. All the documents I can get simply instruct to check the compass using mirrors in the glare shield.
This design decision was used in all the later Douglas models, DC-9, MD-80/90 right upto Boeing 717 (which was developed as Dc-95); incidentally, all these models used a three piece front windshield devoid of center post.


text:
Why is a feather check performed on a piston engine and how much RPM drop should be allowed?
Light general aviation piston twins have full-feathering props. It's normal to perform feathering checks by pulling the prop level all the way down to feather, briefly, and observing the changes. First, why is the feather check performed and what is the pilot looking for during this check? Also, how far should the RPMs be allowed to drop? Could allowing a large RPM drop cause mechanical damage?
I realize that the specific allowances may vary between Lycoming and Continental engines and between models, but an average number would be helpful for the sake of the question.

In my mind the prop check has a lot more to do with being a constant speed prop than being a feathering prop. The check is performed to check control and governor operation and (especially in frigid temps) to cycle warm oil into prop. Most modern designs don't need much of the later.

On singles you don't pull it back to feather so you don't have the possibility of getting that "big RPM drop." But I totally get what you're saying.

My Aztec AFM says do not exceed a 500 RPM max drop for the 1500 RPM feather check and 300 RPM max drop for the 2200 RPM governor check (4.17). In either case you are checking the same control system; the speed of the check determines what part of the control range you are checking.

Nobody seems to have mentioned that you're also checking to make sure you don't get a windshield full of oil when you pull the RPMs back in flight.

@Porcupine911 That's a great point! But if you fly a Cessna 337, like me, you won't get a windshield full of oil when your rear prop governor takes a crap. ;)

The check is performed to make sure the prop will actually feather when pulled past the feather gates.  You'll be in a world of hurt if you are not able to feather a prop if the engine quits.
A couple hundeed RPM drop is acceptable (generally).  So called "deep cycling" props does nothing for you.  I suppose that there are some goofy mechanisms on oddball planes that require a deep cycle but the vast majority only require you to observe a small drop.
Remember, you are checking that the governor can control the prop, not how far the prop can travel.
Edit 1: I should add that you should just do whatever the POH/AFM tells you to do.
Edit 2: a pilot is typically looking for an oil pressure change, manifold pressure rise, and an RPM drop with each prop cycle.

While I voted for this answer, I'd like to add one small correction. I don't know that you'd actually be "in a world of hurt". Your single-engine performance will be significantly degraded, but, even with feathering props, you should still be familiar with single-engine operation of the aircraft when that prop does not feather. The Piper Aztecs I flew forever ago were notorious for not fully feathering. -- EDIT: Oops. Just realized this was an old answer.

Fair point!  A lot of twins will still fly, albeit with a higher Vmc and degraded climb performance.  Change "Aztec" to "Apache" though...

That's sad that Piper put bigger engines on the aircraft but still couldn't figure out how to make them feather properly. Other than the Cub, I was never fond of any of the Pipers I flew. Senecas weren't bad, but I absolutely hated flying the Azwreck. :-/


text:
What is motive flow and why is it used in some aircraft hydraulic systems?
What is motive flow and why is it used in some aircraft hydraulic systems? Why is motive flow used in favor of alternative means of causing fluid flow?
By hydraulic systems, I mean any system that moves a fluid whether it be fuel, oil or some other fluid.

I believe the main advantage is this more simple  than a driven pump, cheap, and needs less maintenance because there is no moving part. The only need is to have an impelling fluid available. Motive flow using Venturi effect with ejectors can be used to create ram air for air-cooled accessories, e.g. electric generators, while the aircraft is on the ground. Bleed air is available, and the pump would be useless mass while in-flight.

Also, all losses end up heating the fuel, a welcome effect when flying near the Stratosphere.

Motive flow is required in design of ejector pumps in fuel system. Ejector pumps (also known as jet pumps or eductor pumps) have no moving parts and therefore represent a highly reliable, low cost means to move fuel in an aircraft. All jet pumps contain the physical characteristics illustrated in below figure.

In this pump, the motive flow comes from a high pressure source such as a primary boost pump, or the returned fuel from the engine. When the high pressure motive flow passes through the throat it creates a suction which sucks fuel from its neighborhood and the total fuel flow are sent to another location. The overall efficiency of an ejector pump is low when compared to electric motor powered pumps but they are frequently used due to their extremely long life, high reliability and low cost.
The ejector pumps generally are used for two functions:

Ejector pumps are often used to provide the primary fuel boost function in smaller transport aircraft applications such as business jets and regional aircraft.
In order to minimize the unusable fuel, scavenge pumps are often employed to suck up fuel from the remote corners of fuel tanks and to discharge this fuel at the inlet to the main feed pump(s). Ejector pumps are used for this purpose and the motive flow for these scavenge devices may be taken from the feed pump outlet if other motive sources are not available.

Note: Most of above material are from: Langton, R. (2009). Aircraft fuel systems. Chichester, U.K Wiley.

Welcome to Aviation.SE, a very nice first answer!